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Polyhedral oligomeric silsesquioxane-reinforced polyurethane acrylate
Progress in Organic Coatings 64 (2009) 205–209
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Polyhedral oligomeric silsesquioxane-reinforced polyurethane acrylate E.H. Kim a , S.W. Myoung a , Y.G. Jung a,∗ , U. Paik b a b
School of Nano & Advanced Materials Engineering, Changwon National University, Changwon, Kyungnam 641-773, Republic of Korea Division of Advanced Materials Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 May 2008 Received in revised form 5 July 2008 Accepted 9 July 2008 Keywords: Nanosized polyhedral oligomeric silsesquioxane (POSS) Organic–inorganic hybrids Elasticity Thermal stability Surface free energy
a b s t r a c t Hybride nanocomposite films of polyhedral oligomeric silsesquioxane (POSS) and polyurethane acrylate (PUA) were prepared by introducing POSS into PUA by free-radical photopolymerization, to enhance thermo-mechanical properties of PUA. The addition of POSS to PUA resulted in increases in the following properties: elasticity, glass transition temperature, thermal stability and dimensional stability. With increasing POSS content, the elastic modulus and thermal stability of PUA films increased due to an increased crosslinking density and the reinforcing effect of POSS particles on the PUA, whereas the surface free energies of these films decreased. The water contact angle against water increased due to the enhancement of the hydrophobicity of the polymer surface, caused by the low surface energy of POSS molecular. However, at high POSS content the mechanical properties of the films were decreased, as a result of aggregation of the POSS particles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polyurethane (PU) is probably the most versatile polymer material, with a wide variety of physical and chemical properties. A PU is prepared from a polyol, an isocyanate and a chain extender [1]. Many PUs are commercially available. Their properties lead to their use in many different applications, for example in building materials, sports goods, medical equipment, adhesives and coatings [1,2]. In addition, these properties can be controlled and improved by various post-treatments such as thermal polymerization and photopolymerization. Ultra-violet (UV) curing is one of well-known photopolymerization methods and employs at various industrial applications. Its advantages include energy savings, and it yields products with high durability and high scratch resistance [3–5]. Over the past few years much attention has been given to improving the thermal and mechanical properties of organic polymers by the incorporation of nanosized inorganic particles [6–8]. Silica (Si) is a most commonly used inorganic particle due to its low price and chemical stability, as well as other factors. However, in conventional experiments in which Si particles are simply blended into organic polymers, the induced aggregation of particles occurs, leading to poor dispersion. Recently, the modification of Si particles by chemical reactions with organic polymers has been used to improve miscibility with the polymer matrix [9]. Specifically, the silanol groups of Si particles have been chemi-
∗ Corresponding author. Tel.: +82 55 279 7622; fax: +82 55 262 6486. E-mail address: [email protected] (Y.G. Jung). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.07.026
cally modified to achieve covalent functionalization. Liu et al. [10] studied the modification of Si particles with epoxy materials and Shirai and Tsubokawa [11] studied reactions with isocyanate groups to graft linear poly(methyl methacrylate) onto Si particles, which resulted in products with a so-called core–shell structure. Recently, polyhedral oligomeric silsesquioxane (POSS) was used to prepare reinforced organic–inorganic hybrids [12,13]. POSS has been used as molecularly surface-modified silica material by organic substituent. This material simplifies the preparation process and improves the properties of the hybrid materials. It also enhances dispersion, which arises from the creation of small-sized particles (1–2 nm), compared to the use of conventional inorganic particles (12 nm in the case of Aerosil 200). Usually, the addition of inorganic particles to a polymer leads to the enhancement of the polymer’s thermal stability, mechanical properties and viscosity, due to the reinforcing effect of the particles. These favorable effects are evident when particles are integrated into a polymer via a chemical reaction. The effect is optimum when the inorganic particles are used as crosslinking agents. It is far preferable to incorporate the inorganic particles into a polymer chain by covalent bonding than by simply mixing them in by shear. Therefore, in our study, nanocomposite PUA–POSS films were prepared by adding the POSS as a crosslinking agent. We added various quantities of POSS to a conventional polyurethane acrylate (PUA) matrix, by free-radical photopolymerization, in efforts to increase the elasticity, glass transition temperature (Tg ), thermal stability and dimensional stability. Properties of the resultant PUA–POSS films were measured using various analytical techniques.
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Table 1 Formulations used to prepare PUA–POSS films, and the resulting hardness values and contact angles of the films. PTMG (mol)
5.88
IPDI (mol)
6.88
HEA (mol)
POSS (pphr)
Functionality (Fav )
Hardness (Shore A)
Contact angle (◦ )
2
0 0.5 1 3 5
2.00 2.18 2.32 2.87 3.32
70 76 79 83 85
48 55 71 89 102
2. Experimental
3. Results and discussion
2.1. Materials and method used to synthesize PUA–POSS nanocomposite films
3.1. Mechanical properties
Urethane acrylate oligomers were prepared from poly(tetramethylene glycol) (PTMG, Mn = 250, Sigma–Aldrich Korea, Yongin, Korea), isophorone diisocyanate (IPDI, extra-pure grade, Sigma–Aldrich Korea, Yongin, Korea), dibutyltin dilaurate (DBT, Sigma–Aldrich Korea, Yongin, Korea) as catalyst, hydroxyethyl acrylate (HEA, Sigma–Aldrich Korea, Yongin, Korea), 2-hydroxy2-methyl-1-phenyl propane-1-one (Darocur 1173, Ciba in Korea, Seoul, Korea) as photoinitiator and octavinyl polyhedral oligomeric silsesquioxane (octavinyl POSS, Sigma–Aldrich Korea, Yongin, Korea). PTMG was dried at 80 ◦ C under 0.1 mm Hg for several hours until bubbling ceased. IPDI was used without further purification. A molar excess of IPDI was reacted with PTMG in N,N dimethylformamide (DMF, Sigma–Aldrich Korea, Yongin, Korea) for 5 h at 60 ◦ C in a four-necked separable flask equipped with a mechanical stirrer, thermometer, condenser and nitrogen inlet tube, to yield a NCO-terminated prepolymer. The reaction mixture was cooled to 40 ◦ C and hydroxyethyl acrylate (HEA, Sigma–Aldrich Korea, Yongin, Korea) was added. A HEA-capped urethane oligomer was obtained. Completion of the reaction was confirmed by the disappearance of the NCO peak at 2270 cm−1 in the FT-IR spectrum. The resulting product was a stable urethane acrylate oligomer with a solids content of about 30%. Finally, POSS particles dispersed in DMF and 4 wt.% Darocur 1173 were fed into the reaction mixture. The product was cast onto a Teflon plate and dried before UV curing. The formulations used to synthesize the different PUA–POSS nanocomposite films are tabulated in Table 1. All PUA films were cured by UV (XL-1000, Spectronics Corporation, NY, USA, 1.5 mW/cm2 , 365 nm) for 3 min.
The hardness value of a material depends primarily on the bulk properties of the material. As the content of the added POSS increases from 0 to 5 pphr, the hardness value (Shore A test) increases from 70 to 85, as shown in Table 1. This implies that POSS has an influence on the bulk properties of films, arising from an increase in the crosslinking density, as a result of reactions between the multifunctional POSS with vinyl groups and PUA. The stress–strain behaviors of PUA–POSS nanocomposite films are shown in Fig. 1. As the content of POSS increases from 0 to 3 pphr various mechanical properties of the films increase, such as the initial elastic modulus, yield strength and tensile strength, while elongation at breaking point decreases, owing to an increase in the crosslinking density as well as the reinforcing effect. POSS particles provide the PUA with rigidity and disturb chain folding, resulting in a high elastic modulus and low strain at breaking point in the glassy state. However, in the case of a POSS content of 5 pphr, aggregation results in a reduction in crosslinking density, which leads to a decrease in tensile strength and initial elastic modulus. Regardless of the POSS content, elongation of a PUA–POSS film at breaking is at least 250%, which is enough for employing the film as a coating material. 3.2. Dynamic mechanical properties The dynamic mechanical behavior of PUA films with different POSS contents is shown in Fig. 2. Regardless of the additional content of POSS, the storage modulus shows a single glass transition,
2.2. Measurements Elongation behaviors of PUA–POSS films were measured with a universal testing apparatus (Ametek, LLOYD Instruments, Hampshire, UK), using a rate of 500 mm/min. Tests were conducted at room temperature, and the averages of at least five runs were calculated. Dynamic mechanical properties of the polymer were measured with a dynamic mechanical thermal analyzer (DMTA MKIV, Rheometrics Scientific, NJ, USA) in tensile mode, using a heating rate of 4 ◦ C/min and 0.02% strain at 10 Hz. A thermogravimetric analyzer (TGA Q50, TA Instruments, West Sussex, USA) was used to measure thermal stability. A sample of about 10 mg of polymer film was placed in a platinum crucible and heated over a temperature range from 30 to 600 ◦ C, at 5 ◦ C/min, under a N2 atmosphere with a flow rate of 40–60 ml/min. Indentation hardness was determined by measuring the Shore A hardness, according to ASTM D2240-75 [14]. Contact angles of films against deionized water were measured using a conventional contact angle gonimeter (G–1, Erma, Tokyo, Japan) at room temperature. Surface morphology of the films was characterized using an atomic force microscope (AFM, Nanocope III, Digital Instruments Inc., NY, USA).
Fig. 1. Stress–strain behavior of PUA films with different POSS contents.
E.H. Kim et al. / Progress in Organic Coatings 64 (2009) 205–209
207
Fig. 2. Storage modulus (E ) of PUA films with different POSS contents. Fig. 3. TGA curves of PUA films with different POSS contents.
which indicates that urethane acrylate and POSS segments of the polymer matrix are phase-mixed at segment level. If two segments are immiscible then, generally, two discrete glass transition peaks are obtained. The Tg and rubbery elastic modulus increase with an increase in the POSS content up to 3 pphr (Tg 81 ◦ C). The PUA chains are chemically bonded to the vinyl groups of POSS during polymerization, indicating that the incorporation of POSS into the PUA interrupts the movement of polymer chains due to the increasing crosslinking density and the reinforcing effect of POSS [15]. Since the polymer has a network structure, as evidenced by the existence of a rubbery plateau, the molecular weight corresponds to the molecular weight between crosslinks (Mc ), which can be calculated based on the ideal rubber theory given by following equation [16]: Mc = 3
RT 0 EN
,
ϕi Fi ,
Fig. 3 shows the thermal stabilities of PUA and of two POSS–PUA nanocomposite films (POSS content 0.5 and 5 pphr). All the films have similar degradation shapes, indicating that the presence of the POSS does not significantly change the degradation of the polymer matrix. Upon the addition of POSS to the PUA, the degradation temperature is slightly increased due to the reinforcing effect of POSS, which implies that POSS can enhance the thermal stability of the PUA films. 3.4. Contact angles
where , R and T are the plateau modulus, density, gas constant and absolute temperature, respectively. Using (1.1 g/cm3 ) and the plateau values tabulated in Fig. 2, Mc was calculated. These values are also tabulated in Fig. 2. It is seen that as the content of POSS increases from 0 to 3 pphr the Mc decreases by more than half, from about 7813 to 3456. Generally, crosslinking density increases as the average functionality of the prepolymer increases and Mc decreases. The average functionality (Fav ) of a prepolymer is determined by the following equation [17]:
3.3. Thermal stability
(1)
0, EN
Fav =
a reduction in crosslinking density, caused by the aggregation of POSS particles during polymerization [18].
(2)
where ϕi is the mole fraction of the oligomer having functionality Fi . The functionality of each of the PUA–POSS films is tabulated in Table 1. However, when the content of POSS is 5 pphr, the Tg and elastic modulus of the film are decreased, in spite of the film having the highest Fav Further, the Mc of the film with a POSS content of 5 pphr is higher than that of the film with 3 pphr. This is due to
Contact angles of water on the surfaces of PUA films are tabulated in Table 1. When the POSS content in the films is increased from 0 to 1 and 5 pphr, the contact angles of water on the surfaces of the PUA films increase from 48◦ to 71◦ and 102◦ , respectively. POSS preferentially migrates toward the surface due to its low molecular weight and low surface energy by hydrophobic vinyl groups, resulting in there tending to be a decrease in the surface energy of the polymer substrate. Incorporation of POSS into polymer chains leads to substantial changes in surface properties, such as water repellency. This in turn means that POSS, by migrating toward the surface, makes the surface much more hydrophobic [19]. In order to minimize the polymer–air surface tension, the material with a lower surface energy is located at the surface. It is well known that such materials with low surface energy provide a thermodynamic driving force for the migration of POSS to the free surface [20–21]. Therefore, POSS can easily migrate to the surface of the polymer. Besides the low surface energy of POSS, the lower molec-
208
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Fig. 4. AFM topographic images of PUA films with different POSS contents: (a) 0 pphr, (b) 0.5 pphr and (c) 3 pphr.
ular weight (Mw ) of POSS also drives the migration. Chain ends have lower surface free energy than the main chain segment, which segregates chain ends at the surface. This effect can usually be obtained with POSS of low Mw rather than PUA with high Mw . According to the following equation, the diffusion coefficient (D) is inversely proportional to Mw [22]: D ∝ 1/(Mw )2 .
(3)
Therefore, POSS having a relatively low Mw and surface energy locates more easily in the surface of the polymer than in the PUA segment, which leads to the enhancement of the contact angles of PUA–POSS films. 3.5. Morphology Fig. 4 shows three-dimensional AFM images of PUA films with different POSS contents. Films containing POSS show a rough surface morphology, while the pure PUA film has much smoother appearance. This is because the lower surface energy of POSS molecular with hydrophobic vinyl groups creates surface roughness, resulting in an increase in the surface area. Such a surface morphology enhances water resistance as well as friction resistance. 4. Conclusions POSS was incorporated into PUA by free-radical photopolymerization to prepare PUA–POSS hybrid nanocomposite films. The yield strength of the films increases when the POSS content is increased: yield strength was about 7 and 11 MPa for films with 0 and 3 pphr
POSS, respectively. Elongation at breaking point decreases when the POSS content is increased. This implies that when POSS is used as a crosslinking agent, it induces a high crosslinking density of the polymer matrix, resulting in restriction of the mobility and flexibility of the polymer chain. As the POSS content increases up to 3 pphr, the Tg and rubbery modulus increase. This results from hindrance of the motion of the polymer chain due to the enhancement of crosslinking density as well as the reinforcing effect of POSS. However, the decrease of tensile strength, elastic modulus and Tg of the nanocomposite film with the high POSS content (5 pphr) can be interpreted in terms of the aggregation of POSS particles reducing the crosslinking density. Contact angles decrease with an increase of POSS content. This is caused by migration of POSS toward the film surface, due to the low molecular weight and low surface energy of POSS having hydrophobic vinyl groups. When the POSS content increases, the roughness of the surface increases, due to the low surface energy of the PUA–POSS films. Acknowledgments The authors acknowledge the financial support received from the Korea Energy Management Corporation (KEMCO) through the Korea Research Foundation Grant (KRF–2006–005–J02701). References [1] C. Hapburn, Polyurethane Elastomers, Elsevier, Oxford, 1991. [2] N.M.K. Lamba, K.A. Woodhous, S.L. Cooper, Polyurethane in Biomedical Applications, CRC Press, New York, 1998. [3] C. Deker, Macromol. Rapid Commun. 23 (2002) 1067. [4] B.K. Kim, S.H. Paik, J. Polym. Sci., Polym. Chem. 37 (1999) 2703. [5] B.K. Kim, B.U. Ahn, M.H. Lee, S.K. Lee, Prog. Org. Coat. 55 (2006) 194.
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L. Chu, M.W. Daniels, L.F. Francis, Chem. Mater. 9 (1997) 2577. P.J. Dionne, R. Ozisik, C.R. Picu, Macromolecules 38 (2005) 9351. F.D. Blum, E.N. Young, G. Smith, O.C. Sitton, Langmuir 22 (2006) 4741. A. Kohut, A. Voronor, W. Peuket, Langmuir 23 (2007) 504. Y.L. Liu, C.Y. Hsu, M.L. Wang, H.S. Chen, Nanotechnology 14 (2003) 813. Y. Shirai, N. Tsubokawa, J. Poly. Sci., Polym. Chem. 39 (2001) 2157. D. Neumann, M. Fisher, L. Tran, J.G. Matisons, J. Am. Chem. Soc. 124 (2002) 13998. [13] S. Zhang, Q. Zou, L. Wu, Macromol. Rapid Commun. 291 (2006) 895. [14] M.H. Lee, H.Y. Choi, K.Y. Jeong, J.W. Lee, T.W. Hwang, B.K. Kim, Polym. Degrad. Stabil. 92 (2007) 1677.
[15] [16] [17] [18] [19] [20] [21] [22]
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B. Yang, H. Xu, J. Wang, S. Gang, C. Li, J. Appl. Polym. Sci. 106 (2007) 320. N.G. Alan, Engineering with Rubber, Hanser, New York, 1992. M. Angel, Polymer Science Dictionary, Chapman & Hall, New York, 1997. C.C. Yang, F.C. Chang, Y.Z. Wang, C.M. Chan, C.C. Lin, W.Y. Chen, J. Polym. Res. 14 (2007) 431. B.S. Kim, S.H. Park, B.K. Kim, Colloid Polym. Sci. 284 (2006) 1067. Y. Yuan, M.S. Shoichet, Macromolecules 33 (2000) 4926. H. Lee, L.A. Archer, Macromolecules 34 (2001) 4572. D.G. Baird, D.I. Collias, Polymer Processing Principles and Design, Wiley, New York, 1998.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Polyhedral oligomeric silsesquioxane-reinforced polyurethane acrylate E.H. Kim a , S.W. Myoung a , Y.G. Jung a,∗ , U. Paik b a b
School of Nano & Advanced Materials Engineering, Changwon National University, Changwon, Kyungnam 641-773, Republic of Korea Division of Advanced Materials Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 May 2008 Received in revised form 5 July 2008 Accepted 9 July 2008 Keywords: Nanosized polyhedral oligomeric silsesquioxane (POSS) Organic–inorganic hybrids Elasticity Thermal stability Surface free energy
a b s t r a c t Hybride nanocomposite films of polyhedral oligomeric silsesquioxane (POSS) and polyurethane acrylate (PUA) were prepared by introducing POSS into PUA by free-radical photopolymerization, to enhance thermo-mechanical properties of PUA. The addition of POSS to PUA resulted in increases in the following properties: elasticity, glass transition temperature, thermal stability and dimensional stability. With increasing POSS content, the elastic modulus and thermal stability of PUA films increased due to an increased crosslinking density and the reinforcing effect of POSS particles on the PUA, whereas the surface free energies of these films decreased. The water contact angle against water increased due to the enhancement of the hydrophobicity of the polymer surface, caused by the low surface energy of POSS molecular. However, at high POSS content the mechanical properties of the films were decreased, as a result of aggregation of the POSS particles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polyurethane (PU) is probably the most versatile polymer material, with a wide variety of physical and chemical properties. A PU is prepared from a polyol, an isocyanate and a chain extender [1]. Many PUs are commercially available. Their properties lead to their use in many different applications, for example in building materials, sports goods, medical equipment, adhesives and coatings [1,2]. In addition, these properties can be controlled and improved by various post-treatments such as thermal polymerization and photopolymerization. Ultra-violet (UV) curing is one of well-known photopolymerization methods and employs at various industrial applications. Its advantages include energy savings, and it yields products with high durability and high scratch resistance [3–5]. Over the past few years much attention has been given to improving the thermal and mechanical properties of organic polymers by the incorporation of nanosized inorganic particles [6–8]. Silica (Si) is a most commonly used inorganic particle due to its low price and chemical stability, as well as other factors. However, in conventional experiments in which Si particles are simply blended into organic polymers, the induced aggregation of particles occurs, leading to poor dispersion. Recently, the modification of Si particles by chemical reactions with organic polymers has been used to improve miscibility with the polymer matrix [9]. Specifically, the silanol groups of Si particles have been chemi-
∗ Corresponding author. Tel.: +82 55 279 7622; fax: +82 55 262 6486. E-mail address: [email protected] (Y.G. Jung). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.07.026
cally modified to achieve covalent functionalization. Liu et al. [10] studied the modification of Si particles with epoxy materials and Shirai and Tsubokawa [11] studied reactions with isocyanate groups to graft linear poly(methyl methacrylate) onto Si particles, which resulted in products with a so-called core–shell structure. Recently, polyhedral oligomeric silsesquioxane (POSS) was used to prepare reinforced organic–inorganic hybrids [12,13]. POSS has been used as molecularly surface-modified silica material by organic substituent. This material simplifies the preparation process and improves the properties of the hybrid materials. It also enhances dispersion, which arises from the creation of small-sized particles (1–2 nm), compared to the use of conventional inorganic particles (12 nm in the case of Aerosil 200). Usually, the addition of inorganic particles to a polymer leads to the enhancement of the polymer’s thermal stability, mechanical properties and viscosity, due to the reinforcing effect of the particles. These favorable effects are evident when particles are integrated into a polymer via a chemical reaction. The effect is optimum when the inorganic particles are used as crosslinking agents. It is far preferable to incorporate the inorganic particles into a polymer chain by covalent bonding than by simply mixing them in by shear. Therefore, in our study, nanocomposite PUA–POSS films were prepared by adding the POSS as a crosslinking agent. We added various quantities of POSS to a conventional polyurethane acrylate (PUA) matrix, by free-radical photopolymerization, in efforts to increase the elasticity, glass transition temperature (Tg ), thermal stability and dimensional stability. Properties of the resultant PUA–POSS films were measured using various analytical techniques.
206
E.H. Kim et al. / Progress in Organic Coatings 64 (2009) 205–209
Table 1 Formulations used to prepare PUA–POSS films, and the resulting hardness values and contact angles of the films. PTMG (mol)
5.88
IPDI (mol)
6.88
HEA (mol)
POSS (pphr)
Functionality (Fav )
Hardness (Shore A)
Contact angle (◦ )
2
0 0.5 1 3 5
2.00 2.18 2.32 2.87 3.32
70 76 79 83 85
48 55 71 89 102
2. Experimental
3. Results and discussion
2.1. Materials and method used to synthesize PUA–POSS nanocomposite films
3.1. Mechanical properties
Urethane acrylate oligomers were prepared from poly(tetramethylene glycol) (PTMG, Mn = 250, Sigma–Aldrich Korea, Yongin, Korea), isophorone diisocyanate (IPDI, extra-pure grade, Sigma–Aldrich Korea, Yongin, Korea), dibutyltin dilaurate (DBT, Sigma–Aldrich Korea, Yongin, Korea) as catalyst, hydroxyethyl acrylate (HEA, Sigma–Aldrich Korea, Yongin, Korea), 2-hydroxy2-methyl-1-phenyl propane-1-one (Darocur 1173, Ciba in Korea, Seoul, Korea) as photoinitiator and octavinyl polyhedral oligomeric silsesquioxane (octavinyl POSS, Sigma–Aldrich Korea, Yongin, Korea). PTMG was dried at 80 ◦ C under 0.1 mm Hg for several hours until bubbling ceased. IPDI was used without further purification. A molar excess of IPDI was reacted with PTMG in N,N dimethylformamide (DMF, Sigma–Aldrich Korea, Yongin, Korea) for 5 h at 60 ◦ C in a four-necked separable flask equipped with a mechanical stirrer, thermometer, condenser and nitrogen inlet tube, to yield a NCO-terminated prepolymer. The reaction mixture was cooled to 40 ◦ C and hydroxyethyl acrylate (HEA, Sigma–Aldrich Korea, Yongin, Korea) was added. A HEA-capped urethane oligomer was obtained. Completion of the reaction was confirmed by the disappearance of the NCO peak at 2270 cm−1 in the FT-IR spectrum. The resulting product was a stable urethane acrylate oligomer with a solids content of about 30%. Finally, POSS particles dispersed in DMF and 4 wt.% Darocur 1173 were fed into the reaction mixture. The product was cast onto a Teflon plate and dried before UV curing. The formulations used to synthesize the different PUA–POSS nanocomposite films are tabulated in Table 1. All PUA films were cured by UV (XL-1000, Spectronics Corporation, NY, USA, 1.5 mW/cm2 , 365 nm) for 3 min.
The hardness value of a material depends primarily on the bulk properties of the material. As the content of the added POSS increases from 0 to 5 pphr, the hardness value (Shore A test) increases from 70 to 85, as shown in Table 1. This implies that POSS has an influence on the bulk properties of films, arising from an increase in the crosslinking density, as a result of reactions between the multifunctional POSS with vinyl groups and PUA. The stress–strain behaviors of PUA–POSS nanocomposite films are shown in Fig. 1. As the content of POSS increases from 0 to 3 pphr various mechanical properties of the films increase, such as the initial elastic modulus, yield strength and tensile strength, while elongation at breaking point decreases, owing to an increase in the crosslinking density as well as the reinforcing effect. POSS particles provide the PUA with rigidity and disturb chain folding, resulting in a high elastic modulus and low strain at breaking point in the glassy state. However, in the case of a POSS content of 5 pphr, aggregation results in a reduction in crosslinking density, which leads to a decrease in tensile strength and initial elastic modulus. Regardless of the POSS content, elongation of a PUA–POSS film at breaking is at least 250%, which is enough for employing the film as a coating material. 3.2. Dynamic mechanical properties The dynamic mechanical behavior of PUA films with different POSS contents is shown in Fig. 2. Regardless of the additional content of POSS, the storage modulus shows a single glass transition,
2.2. Measurements Elongation behaviors of PUA–POSS films were measured with a universal testing apparatus (Ametek, LLOYD Instruments, Hampshire, UK), using a rate of 500 mm/min. Tests were conducted at room temperature, and the averages of at least five runs were calculated. Dynamic mechanical properties of the polymer were measured with a dynamic mechanical thermal analyzer (DMTA MKIV, Rheometrics Scientific, NJ, USA) in tensile mode, using a heating rate of 4 ◦ C/min and 0.02% strain at 10 Hz. A thermogravimetric analyzer (TGA Q50, TA Instruments, West Sussex, USA) was used to measure thermal stability. A sample of about 10 mg of polymer film was placed in a platinum crucible and heated over a temperature range from 30 to 600 ◦ C, at 5 ◦ C/min, under a N2 atmosphere with a flow rate of 40–60 ml/min. Indentation hardness was determined by measuring the Shore A hardness, according to ASTM D2240-75 [14]. Contact angles of films against deionized water were measured using a conventional contact angle gonimeter (G–1, Erma, Tokyo, Japan) at room temperature. Surface morphology of the films was characterized using an atomic force microscope (AFM, Nanocope III, Digital Instruments Inc., NY, USA).
Fig. 1. Stress–strain behavior of PUA films with different POSS contents.
E.H. Kim et al. / Progress in Organic Coatings 64 (2009) 205–209
207
Fig. 2. Storage modulus (E ) of PUA films with different POSS contents. Fig. 3. TGA curves of PUA films with different POSS contents.
which indicates that urethane acrylate and POSS segments of the polymer matrix are phase-mixed at segment level. If two segments are immiscible then, generally, two discrete glass transition peaks are obtained. The Tg and rubbery elastic modulus increase with an increase in the POSS content up to 3 pphr (Tg 81 ◦ C). The PUA chains are chemically bonded to the vinyl groups of POSS during polymerization, indicating that the incorporation of POSS into the PUA interrupts the movement of polymer chains due to the increasing crosslinking density and the reinforcing effect of POSS [15]. Since the polymer has a network structure, as evidenced by the existence of a rubbery plateau, the molecular weight corresponds to the molecular weight between crosslinks (Mc ), which can be calculated based on the ideal rubber theory given by following equation [16]: Mc = 3
RT 0 EN
,
ϕi Fi ,
Fig. 3 shows the thermal stabilities of PUA and of two POSS–PUA nanocomposite films (POSS content 0.5 and 5 pphr). All the films have similar degradation shapes, indicating that the presence of the POSS does not significantly change the degradation of the polymer matrix. Upon the addition of POSS to the PUA, the degradation temperature is slightly increased due to the reinforcing effect of POSS, which implies that POSS can enhance the thermal stability of the PUA films. 3.4. Contact angles
where , R and T are the plateau modulus, density, gas constant and absolute temperature, respectively. Using (1.1 g/cm3 ) and the plateau values tabulated in Fig. 2, Mc was calculated. These values are also tabulated in Fig. 2. It is seen that as the content of POSS increases from 0 to 3 pphr the Mc decreases by more than half, from about 7813 to 3456. Generally, crosslinking density increases as the average functionality of the prepolymer increases and Mc decreases. The average functionality (Fav ) of a prepolymer is determined by the following equation [17]:
3.3. Thermal stability
(1)
0, EN
Fav =
a reduction in crosslinking density, caused by the aggregation of POSS particles during polymerization [18].
(2)
where ϕi is the mole fraction of the oligomer having functionality Fi . The functionality of each of the PUA–POSS films is tabulated in Table 1. However, when the content of POSS is 5 pphr, the Tg and elastic modulus of the film are decreased, in spite of the film having the highest Fav Further, the Mc of the film with a POSS content of 5 pphr is higher than that of the film with 3 pphr. This is due to
Contact angles of water on the surfaces of PUA films are tabulated in Table 1. When the POSS content in the films is increased from 0 to 1 and 5 pphr, the contact angles of water on the surfaces of the PUA films increase from 48◦ to 71◦ and 102◦ , respectively. POSS preferentially migrates toward the surface due to its low molecular weight and low surface energy by hydrophobic vinyl groups, resulting in there tending to be a decrease in the surface energy of the polymer substrate. Incorporation of POSS into polymer chains leads to substantial changes in surface properties, such as water repellency. This in turn means that POSS, by migrating toward the surface, makes the surface much more hydrophobic [19]. In order to minimize the polymer–air surface tension, the material with a lower surface energy is located at the surface. It is well known that such materials with low surface energy provide a thermodynamic driving force for the migration of POSS to the free surface [20–21]. Therefore, POSS can easily migrate to the surface of the polymer. Besides the low surface energy of POSS, the lower molec-
208
E.H. Kim et al. / Progress in Organic Coatings 64 (2009) 205–209
Fig. 4. AFM topographic images of PUA films with different POSS contents: (a) 0 pphr, (b) 0.5 pphr and (c) 3 pphr.
ular weight (Mw ) of POSS also drives the migration. Chain ends have lower surface free energy than the main chain segment, which segregates chain ends at the surface. This effect can usually be obtained with POSS of low Mw rather than PUA with high Mw . According to the following equation, the diffusion coefficient (D) is inversely proportional to Mw [22]: D ∝ 1/(Mw )2 .
(3)
Therefore, POSS having a relatively low Mw and surface energy locates more easily in the surface of the polymer than in the PUA segment, which leads to the enhancement of the contact angles of PUA–POSS films. 3.5. Morphology Fig. 4 shows three-dimensional AFM images of PUA films with different POSS contents. Films containing POSS show a rough surface morphology, while the pure PUA film has much smoother appearance. This is because the lower surface energy of POSS molecular with hydrophobic vinyl groups creates surface roughness, resulting in an increase in the surface area. Such a surface morphology enhances water resistance as well as friction resistance. 4. Conclusions POSS was incorporated into PUA by free-radical photopolymerization to prepare PUA–POSS hybrid nanocomposite films. The yield strength of the films increases when the POSS content is increased: yield strength was about 7 and 11 MPa for films with 0 and 3 pphr
POSS, respectively. Elongation at breaking point decreases when the POSS content is increased. This implies that when POSS is used as a crosslinking agent, it induces a high crosslinking density of the polymer matrix, resulting in restriction of the mobility and flexibility of the polymer chain. As the POSS content increases up to 3 pphr, the Tg and rubbery modulus increase. This results from hindrance of the motion of the polymer chain due to the enhancement of crosslinking density as well as the reinforcing effect of POSS. However, the decrease of tensile strength, elastic modulus and Tg of the nanocomposite film with the high POSS content (5 pphr) can be interpreted in terms of the aggregation of POSS particles reducing the crosslinking density. Contact angles decrease with an increase of POSS content. This is caused by migration of POSS toward the film surface, due to the low molecular weight and low surface energy of POSS having hydrophobic vinyl groups. When the POSS content increases, the roughness of the surface increases, due to the low surface energy of the PUA–POSS films. Acknowledgments The authors acknowledge the financial support received from the Korea Energy Management Corporation (KEMCO) through the Korea Research Foundation Grant (KRF–2006–005–J02701). References [1] C. Hapburn, Polyurethane Elastomers, Elsevier, Oxford, 1991. [2] N.M.K. Lamba, K.A. Woodhous, S.L. Cooper, Polyurethane in Biomedical Applications, CRC Press, New York, 1998. [3] C. Deker, Macromol. Rapid Commun. 23 (2002) 1067. [4] B.K. Kim, S.H. Paik, J. Polym. Sci., Polym. Chem. 37 (1999) 2703. [5] B.K. Kim, B.U. Ahn, M.H. Lee, S.K. Lee, Prog. Org. Coat. 55 (2006) 194.
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